The Moon

Formation

Lunar Facts• The moon ended its formation period approximately 4

billion years ago. • After the period of formation, the surface of the moon

continued to be heavily bombarded by the remains of planetary materials. This period is common to all planets and is referred to as the period of Late, Heavy Bombardment. During this period, the moon warmed, separated into a core and mantle, and experienced volcanism. Toward the end of this period the moon is thought to have experienced a series of significant collisions which formed the lunar Maria.

• Due to its small size, the moon should have cooled very rapidly compared to earth. All surface activity in the form of plate tectonics would have ceased once the moon had cooled. Even today, the interior of the moon appears to have cooled to a point of complete inactivity.

Factors to be taken into account when hypothesizing about

formation of the Moon• the moon’s low density of 3.3 g/cc proves

that the moon does not have a significant iron core like the earth does,

• the moon’s rocks contain very few volatile substances which implies it was heated more than the earth

• the earth and the moon have identical values for the relative abundance of oxygen isotopes, which implies that the Earth and the Moon formed at the same distance from the sun.

The Five Major Theories of Formation of the

Earth’s Moon

Evidence supporting the Co-accretion Theory

• States that the earth and the moon accreted at the same time out of the same nebular material

• In this theory, the proto-moon drew material out of the same nebular cloud as the earth in the same relative location as result, the two should be very similar in composition

• Why it doesn’t work: The co-formation theory explains why the moon is located in its current location, but cannot explain the evidence that the earth and moon are composed of different materials.

Co-accretion Image

Did the moon form by Fission?

• Theory proposed by Darwin

• Based on fast-spinning primordial earth

• Earth spun and flattened so quickly that it ejected a large

piece of material, which eventually became the moon

• Strengths: Isotopic ratio and Iron content similarities

between Earth and Moon are explained

• Flaws: Energy needed to cause loss of the material not

supported by present day spinning of the earth

The Capture Theory

• Ring of dust around

the earth slows the

moon, which has

already formed,

allowing it to be

captured into the

earth’s gravitational

field.

Capture Theory Continued…• The Capture theory postulates that the moon

was formed at another place and time in the solar system and while passing by the earth, it was pulled into the earth’s gravitational field.

• Reasonable hypothesis because many moons surrounding other planets are actually captured asteroids and not objects that formed in place with the mother planet. A moon that is captured would most likely have a non-spherical shape. Ex. Phobos and Deimos, the moons of Mars

The Moons of Mars (captured satellites)

Why the Capture theory does not work

• indicator that a moon has been captured would be

if it orbited in a direction that differed from that of

the mother planet, but our moon is rounded in

shape and orbits the earth, the capture theory

does not hold up.

• The only piece of evidence supported by the

capture theory is the difference in composition

between the earth and the moon.

Colliding Planetesimals

• Hypothesizes that the moon condensed

from the debris of planetesimaly sized

objects that collided during the

formation of the solar system

• Limited evidence to support this theory

The One that Works!

The Collision-Ejection Theory

aka: The Giant Impactor Theory

What this theory is all about

• Hypothesizes that the moon was formed

when a planetesimal the size of mars struck

the earth, thereby ejecting large volumes of

matter from the earth.

• The disk of orbiting material ejected from

the collision eventually condensed to form

our moon in its orbit around the Earth.

Origin of the Theory• The theory was proposed in the mid-1970’s, but

was rejected by many scientists until 1984 when

a conference evaluating the validity of theories

of the moon left no doubt that the collision

theory was the most likely possibility

• New models of planet formation had suggested

that giant impacts were not at all uncommon

during the late stages of terrestrial planet

formation.

Collision Theory Images

Why this theory works

Explains the Lack of volatiles on the Moon’s surface

• In order to explain the lack of volatiles on

the moon, we would need an event which created

a heat so high that all would have been vaporized.

If an object the size of Mars were to collide with

the forming Earth, the heat produced by this

collision would provide a reasonable explanation

as to why the moon’s surface characteristics imply

that it has been ‘baked’ more than the earth.

The Iron Core of the Moon explained

• moon is also thought to contain a small iron core

• The collision theory states that the moon would be

able to retain the iron core even through the collision.

• The earth’s Iron core had already undergone

gravitational differentiation at the time of the impact,

therefore, the debris ejected from the earth consisted

of material from the iron depleted rocky mantle.

Supports similarities in Oxygen isotopic ratios

• The earth and the moon have exactly the

same oxygen isotope composition while

rocks and meteorites from different areas

of the solar system have differing ratios.

• The similarity in isotopic ratios supports

the belief that the moon formed from

material in the earth’s vicinity.

How do we know about the moon?

These

Guys

Brought

back this

With this

Lunar Structure

• Differentiated

• Very small core

Lunar Composition

• Lacking in metal (25% of radius vs 50%

on earth)

• Crust formed from a “Magma Ocean”

• Composition is similar

to Earth’s Mantle

Check it

• 4.5 billion years of lunar evolution

compressed into 3 minutes

• http://lunarscience.nasa.gov/articles/

video-evolution-of-the-moon

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